Optical glass

ABSTRACT

An optical glass having a refractive index (n d ) and an Abbe number (v d ) which are within an area surrounded by the straight lines which are drawn by connecting point A (n d =1.835, v d =46.5), point B (n d =1.90, v d =40.0), point C, (n d =1.90, v d =35.0) and point D (n d =1.835, v d =38.0) in a sequence of A, B, C, D and A as border lines in x-y coordinates shown in FIG.  1,  in which X-axis is the Abbe number (v d ) and Y-axis is the refractive index (n d ), the area including the border line. The optical glass has low glass transition temperature (Tg), and suitable for precision mold pressing. The optical glass which has the refractive index (n d ) and Abbe number (v d ) within the above-described area, where the area includes the border lines, has the composition of SiO 2 —B 2 O 3 —La 2 O 3 —Gd 2 O 3 —Li 2 O—F system, the transition temperature (Tg) of 550 to 650° C., and is free from lead, cadmium, thorium, Y 2 O 3 , P 2 O 5  and TeO 2 .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a glass preform material used inprecision mold pressing and an optical glass suitable for precision moldpressing, which have low dispersion and high refractive index and lowtransition point (Tg).

[0003] 2. Description of Related Art

[0004] In late years, in optical instruments remarkably getting smallerand lighter, an aspherical lens is often applied in order to decreasethe number of lenses constituting an optical system of an opticalinstrument. A method in which a lens is produced by press forming aheat-softened glass preform material with a metal molding die havingprecise molding surface, or a precision mold pressing, is a mainstreamof manufacturing a glass aspherical lens.

[0005] Since the glass molding obtained by precision mold pressing isavailable as final products without or almost without grinding andpolishing after forming, a lens and the like can be manufactured in highproductivity with precision mold pressing. Precision mold pressingmethod is a suitable method for manufacturing a glass aspherical lensbecause it is difficult to manufacture an aspherical lens in largeamount at a low cost in a former method including grinding andpolishing. Furthermore, a precision mold pressing method has highproductivity. As a result, spherical lenses and other optical elementsof various shapes as well as an aspherical lens are manufactured in aprecision mold pressing.

[0006] There are two major methods of manufacturing the glass preformmaterial for precision glass forming.

[0007] One of these methods is: as disclosed in Japanese PatentApplication Publication (Unexamined) Tokukaihei-6-122526 andTokukaihei-8-319124, flowing molten glass from a discharge pipe, cuttingthe flowed glass while the flowed glass keeps on a molding die, andsuccessively forming glass perform material; or, as disclosed inJapanese Patent Application Publication (Unexamined)Tokukaihei-7-165431, dropping molten glass from a flow-controlled pipe,keeping the droplets on lower molding die, and forming a glass preformmaterial by pressing the kept glass lump with upper molding die. Thesemethods have high productivity of a glass preform material itselfbecause a glass preform material is directly obtained by hot forming ofmolten glass in both methods.

[0008] The other method is one obtaining a glass preform material bycutting a glass block material. There is a problem in this method thatmany processes are required from cutting a glass block material toshaping a glass close to a final product. However, there is an advantagethat shape variation in precision mold pressing can be reduced on aoccasion of obtaining a final product of various shapes such as a lensby precision mold pressing since the glass preform material can beshaped close to a final product such as a lens.

[0009] In obtaining a glass molding with precision mold pressing, aheat-softened glass preform material is required to be press formedunder high temperature condition in order to print precise moldingsurface of metal molding die to the glass preform material. Therefore,the applied metal molding die is also exposed to high temperature, andfurthermore, to high pressure. As a result, when a glass preformmaterial is heat-softened and press-formed, a precise molding surface ofmetal molding die often cannot be maintained because of oxidation anderosion of the molding surface and a damage of a mold release membraneprovided on the molding surface. Further, the metal molding die itselfis damaged easily. Under these conditions, since the metal molding dieis required to exchange frequently, the frequency of exchanging themetal molding die increases as a result, the manufacturing in largeamount at low cost cannot to be achieved. The transition temperature(Tg) of a glass for a glass preform material used in precision moldpressing is desirably as low as possible from the viewpoints of reducingthe above-described damage, maintaining precise molding surface of ametal molding die for a long period and permitting precision moldpressing with low pressure. At present, since a glass of the glasspreform material for precision mold pressing having transition point(Tg) of more than 650° C. is difficult to be applied to precision moldpressing, a high refractive index and low dispersion glass havingtransition temperature (Tg) of not more than 650° C. is required,preferably the transition temperature (Tg) of not more than 640° C.,more preferably the transition temperature (Tg) of not more than 630° C.Further, since a devitrification of the glass preform material does notdisappear by precision mold pressing, and since a devitrified glassmolding cannot be used as an optical element such as a lens, the glassof a glass preform material for precision mold pressing is required tohave superior resistance to devitrification property.

[0010] As for the property of an optical glass used for an asphericallens, various optical constants (refractive index (n_(d)) and Abbenumber (v_(d))) are required. Among them, the glasses having highrefractive index and low dispersion are required in recent years.Particularly, for an optical design, high refractive index and lowdispersion glass is strongly required, wherein a refractive index(n_(d)) and an Abbe number (v_(d)) are within an area surrounded bystraight lines that are drawn by connecting point A (n_(d)=1.835,v_(d)=46.5), point B (n_(d)=1.90, v_(d)=40.0), point C, (n_(d)=1.90,v_(d)=35.0) and point D (n_(d)=1.835, v_(d)=38.0) in a sequence of A, B,C, D and A as border lines in x-y orthogonal coordinates shown in FIG.1, in which X-axis is the Abbe number (v_(d)) and Y-axis is therefractive index (n_(d)), the area including the border lines(hereinafter, the above-described area is referred to as a specificarea). In particular, an optical glass having the optical constantswithin the specific area and refractive index (n_(d)) more than 1.85 andless than 1.875, an optical glass having the optical constants withinthe specific area and Abbe number (v_(d)) less than 39.5 and the likeare required.

[0011] Cadmium and thorium components have been known for a long time asingredients providing high refractive index to a glass. However, it iswell known that these ingredients cause environmental damage.

[0012] A glass containing lead such as PbO is known as a glass havinghas a high refractive index and a low transition temperature (Tg). Sincea glass containing lead easily fuses with a metal molding die inprecision mold pressing, it is difficult to use a metal molding dierepeatedly. Therefore, it is not suitable for a glass of precision moldpressing. Furthermore, for the sake of unfavorable environmental effectof lead, there is a problem that some kinds of environmental measuresare required in manufacturing, coolworking such as polishing, disposaland the like of a glass containing lead. There is also a movement toprohibit manufacture, usage and the like of a glass containing lead.

[0013] For these reason, an optical glass having high refractive indexand low dispersion, low transition temperature, and containing no leadis strongly required.

[0014] Since a high refractive index and low dispersion optical glass isuseful for a spherical lens as well as an aspherical lens from aviewpoint of optical design, various kinds of the glasses have beensuggested for long. For example, Japanese Patent Application Publication(Unexamined) Tokukaisho-52-14607 (hereinafter referred to asPublication 1) discloses a B₂O₃—SiO₂—La₂O₃—Gd₂O₃—ZrO₂+Ta₂O₅ systemoptical glass. The glass disclosed in Publication 1 does not fulfill therecent above-described requirement for optical design, because therefractive index is low and the optical constants is not within theabove-described specific area. Further it is difficult to perform theprecision mold pressing for the sake of its high transition temperature(Tg).

[0015] Japanese Patent Application Publication (Unexamined)Tokukaisho-52-155614 (hereinafter referred to as Publication 2)discloses a B₂O₃—La₂O₃—Gd₂O₃—WO₃—ZrO₂+Ta₂O₅ system high refractive indexand low dispersion optical glass. Though the optical glass disclosed inPublication 2 has the optical constants within the above-describedspecific area, it is difficult to perform the precision mold pressingfor the sake of its high transition temperature (Tg).

[0016] Japanese Patent Application Publication (Examined)Tokukousho-54-2646 discloses a SiO₂—B₂O₃—La₂O₃—Ta₂O₅+ZnO system highrefractive index and low dispersion optical glass. It is difficult thatthe optical glass disclosed on this publication is applied to precisionmold pressing for the sake of its high transition temperature (Tg).

[0017] Japanese Patent Application Publication (Examined)Tokukousho-53-4023 discloses a B₂O₃—La₂O₃—HfO₂ system high refractiveindex and low dispersion optical glass. It is difficult that the opticalglass disclosed on this publication is applied to precision moldpressing for the sake of its high transition temperature (Tg).

[0018] Japanese Patent Application Publication (Unexamined)Tokukaisho-54-90218 (hereinafter referred to as Publication 3) disclosesa SiO₂—B₂O₃—La₂O₃—WO₃—Ta₂O₅—Gd₂O₃—ZrO₂ system high refractive index andlow dispersion optical glass. It is difficult that the glass practicallydisclosed in Publication 2 is applied to precision mold pressing for thesake of its high transition temperature (Tg).

[0019] Japanese Patent Application Publication (Unexamined)Tokukaisho-52-129716 and Japanese Patent Application Publication(Examined) Tokukousho-54-6042 (hereinafter referred to as Publication 4)discloses a B₂O₃—La₂O₃—Y₂O₃—ZrO₂—WO3—Ta₂O₅+Nb₂O₅+TiO₂ system highrefractive index and low dispersion optical glass. It is difficult thatboth of the glasses disclosed in these publications are applied toprecision mold pressing for the sake of their high transitiontemperature (Tg).

[0020] Japanese Patent Application Publication (Unexamined)Tokukaisho-60-46948 discloses a SiO₂—B₂O₃—La₂O₃—Yb₂O₃—Nb₂O₅—Ta₂O₅ systemhigh refractive index and low dispersion optical glass. It is difficultthat the glass practically disclosed in this publication is applied toprecision mold pressing for the sake of its high transition temperature(Tg).

[0021] Japanese Patent Application Publication (Unexamined)Tokukaisho-60-221338 discloses a B₂O₃—La₂O₃—Y₂O₃-bivalent metaloxide-Li₂O system optical glass having low transition temperature (Tg).Since the glass practically disclosed on this publication does not havethe optical constants within the above-described specific area, it doesnot fulfill the above-described recent requirement for optical design.

[0022] Japanese Patent Application Publication (Unexamined)Tokukaisho-62-100449 discloses a B₂O₃—La₂O₃—ZnO—Li₂O—Sb₂O₃ systemoptical glass having low transition temperature (Tg). Since this glassdisclosed on this publication contains much Sb₂O₃ as an essentialcomponent, in obtaining a glass preform material in hot forming,selective volatilization of Sb₂O₃ component form the surface layer offused glass tend to cause the surface striae. The surface striae doesnot disappear by precision mold pressing, and a glass molding havingsurface striae does not used for optical elements such as lenses.Therefore, it is not suitable for the glass preform material forprecision mold pressing. Further, on precision mold pressing of a glasspreform material, Sb₂O₃ component selectively volatilized from thesurface layer of a glass preform material adheres to the molding surfaceof a metal molding die to cause haze on the molding surface of a metalmolding die. If the hazed metal die is used repeatedly, the haze itselfis transferred to the surface of a glass molding, or the haze pattern isprinted to the surface of a glass molding. Since a glass molding withthese problems cannot be used as an optical element such as a lens, itis not suitable for mass production of optical elements such as lensesby precision mold pressing. Furthermore, since the glass practicallydisclosed on this publication does not have the optical constants withinthe above-described specific area, it does not fulfill theabove-described recent requirement for optical design.

[0023] Japanese Patent Application Publication (Unexamined)Tokukaihei-8-217484 (hereinafter referred to as Publication 5) disclosesa B₂O₃—La₂O₃—Lu₂O₃ system optical glass. Since the Lu₂O₃ componentcontained in this glass as essential component is remarkably expensive,this glass has little practical use because of very high productioncost. Among the glasses disclosed on Publication 5, it is difficult thatthe glasses having the optical constants within the above-describedspecific area is applied to precision mold pressing for the sake of hightransition temperature (Tg).

[0024] Japanese Patent Application Publication (Unexamined)Tokukai-2001-348244 (hereinafter referred to as Publication 6) disclosesoptical glass having glass transition point of not more than 700° C. andoptical property of high refractive index and low dispersion. It isdifficult that a glass practically disclosed on Publication 6 is appliedto precision mold pressing for the sake of high glass transition point(Tg), or transition temperature (Tg).

[0025] Japanese Patent Application Publication (Unexamined)Tokukai-2003-267748 (hereinafter referred to as Publication 7) disclosesa high refractive index and low dispersion optical glass having low Tg.The glass practically disclosed in Publication 7 having the opticalconstants within the above-described specific area has a defect ofhaving a difficulty in manufacturing a glass preform material for thesake of its low resistance to devitrification property. Further, thereis also a defect that transmission factor in short visible wavelengthrange is low since a lot of WO₃ and TiO₂ is contained.

[0026] As described above, high refractive index and low dispersionoptical glasses according to an earlier development have problems mainlythat: though transition temperature is low, its optical constants is notwithin the above-described specific area strongly required in lateyears; or that, though its optical constants is within theabove-described specific area, the transition temperature (Tg) is high,which cause a difficulty of precision mold pressing.

SUMMARY OF THE INVENTION

[0027] An object of the present invention is to resolve the defects ofthe optical glass described in Description of Related Art, and toprovide an optical glass which has optical constants within theabove-described specific area and a low transition temperature (Tg), andwhich is suitable for a glass preform material for precision moldpressing and precision mold pressing, and furthermore, is free fromcadmium component, thorium component and lead component in order to havelow environmental burden.

[0028] In order to accomplish the above-described object, the inventorshas examined and researched an optical lens. As a result, the inventorshas found an optical glass which has optical constants in a specificrange and low transition temperature (Tg) at which precise press formingcan be performed, and which is suitable for a glass preform material forprecision mold pressing and precision mold pressing, and is availablefor precision mold pressing. Thus, the inventor has accomplished thefollowing invention.

[0029] According to a first aspect of the present invention, an opticalglass has;

[0030] a refractive index (n_(d)) and an Abbe number (v_(d)) which arewithin an area surrounded by straight lines that are drawn by connectingpoint A (n_(d)=1.835, v_(d)=46.5), point B (n_(d)=1.90, v_(d)=40.0),point C, (n_(d)=1.90, v_(d)=35.0) and point D (n_(d)=1.835, v_(d)=38.0)in a sequence of A, B, C, D and A as border lines in x-y orthogonalcoordinates shown in FIG. 1, in which X-axis is the Abbe number (v_(d))and Y-axis is the refractive index (n_(d)), the area including theborder lines: and the optical glass comprises:

[0031] 0.1 to 8 mass % of SiO₂;

[0032] 5 to less than 20 mass % B₂O₃;

[0033] 15 to 50 mass % of La₂O₃;

[0034] 0.1 to 30 mass % Gd₂O₃,

[0035] 0 to 10 mass % of GeO₂ and

[0036] 0 to 8 mass % of Nb₂O₅,

[0037] where a total content of Gd₂O₃, GeO₂ and Nb₂O₅ is more than 10mass % to 30 mass %;

[0038] 0 to 5 mass % of Yb₂O₃;

[0039] 0 to 1 mass % of TiO₂;

[0040] 0 to 8 mass % of ZrO₂;

[0041] more than 10 to 25 mass % of Ta₂O₅;

[0042] 0 to 10 mass % of WO₃;

[0043] 0 to 15 mass % of ZnO;

[0044] 0 to 5 mass % of RO,

[0045] where RO is one or more kinds of oxides selected from CaO, SrOand BaO;

[0046] more than 0.5 to less than 3 mass % of Li₂O;

[0047] 0.1 to 1 mass % of Sb₂O₃; and

[0048] 1 to 6 mass % a the total content of fluorides of above-describedmetal elements as F element with which a part or all of one or morekinds of oxides of above-described metal elements are substituted;

[0049] wherein the optical glass is free from cadmium, thorium Y₂O₃,P₂O₅, and TeO₂, and

[0050] the optical glass has a transition temperature (Tg) of 550 to650° C.

[0051] The above-described optical glass preferably comprises 0.1 toless than 5.5% of SiO₂.

[0052] The above-described optical glass preferably comprised more than1 to less than 3% of Li₂O.

[0053] The above-described optical glass may have the refractive index(n_(d)) of less than 1.875.

[0054] The above-described optical glass may have the refractive index(n_(d)) of 1.875 or more.

[0055] The above-described optical glass may have the refractive index(n_(d)) of more than 1.85.

[0056] The above-described optical glass may have the Abbe number(v_(d)) of less than 39.5.

[0057] The above-described optical glass may have the Abbe number(v_(d)) of 39.5 and more.

[0058] The above-described optical glass preferably has the transitiontemperature (Tg) of 640° C. or less.

[0059] The above-described optical glass preferably has the transitiontemperature (Tg) of 630° C. or less.

[0060] According to the second aspect of the present invention, anoptical glass has

[0061] a refractive index (n_(d)) and an Abbe number (v_(d)) which arewithin an area surrounded by straight lines that are drawn by connectingpoint A (n_(d)=1.835, v_(d)=46.5), point B (n_(d)=1.90, v_(d)=40.0),point C, (n_(d)=1.90, v_(d)=35.0) and point D (n_(d)=1.835, v_(d)=38.0)in a sequence of A, B, C, D and A as border lines in x-y orthogonalcoordinates shown in FIG. 1, in which X-axis is the Abbe number (v_(d))and Y-axis is the refractive index (n_(d)), the area including theborder lines: and the optical glass comprises;

[0062] 0.1 to 8 mass % of SiO₂;

[0063] 5 to less than 20 mass % B₂O₃;

[0064] 15 to 50 mass % of La₂O₃;

[0065] 0.1 to 30 mass % Gd₂O₃,

[0066] more than 10 to 25 mass % of Ta₂O₅; and

[0067] more than 0.5 to less than 3 mass % of Li₂O; and

[0068] 0 to 10 mass % of GeO₂ and/or

[0069] 0 to 8 mass % of Nb₂O₅,

[0070] where a total content of Gd₂O₃, GeO₂ and Nb₂O₅ is more than 10mass % to 30 mass %; and/or

[0071] 0 to 5 mass % of Yb₂O₃; and/or

[0072] 0 to 1 mass % of TiO₂; and/or

[0073] 0 to 8 mass % of ZrO₂; and/or

[0074] 0 to 10 mass % of WO₃; and/or

[0075] 0 to 15 mass % of ZnO; and/or

[0076] 0 to 5 mass % of RO,

[0077] where RO is one or more kinds of oxides selected from CaO, SrOand BaO;

[0078] 0 to 1 mass % of Sb₂O₃; and/or

[0079] 0 to less than 0.5 mass % of Lu₂O₃; and

[0080] 0.1 to 6 mass % of the total content of fluorides ofabove-described metal elements as F element with which a part or all ofone or more kinds of oxides of above-described metal elements aresubstituted;

[0081] wherein the optical glass is free from cadmium, thorium Y₂O₃,P₂O₅ and TeO₂, and

[0082] the optical glass has a transition temperature (Tg) of 550 to650° C.

[0083] The above-described optical glass preferably comprises 0.1 toless than 5.5% of SiO₂.

[0084] The above-described optical glass preferably comprises more than1 to less than 3% of Li₂O.

[0085] The above-described optical glass may have the refractive index(n_(d)) of less than 1.875.

[0086] The above-described optical glass may have the refractive index(n_(d)) of 1.875 or more.

[0087] The above-described optical glass may have the refractive index(n_(d)) of more than 1.85.

[0088] The above-described optical glass may have the Abbe number(v_(d)) of less than 39.5.

[0089] The above-described optical glass may have the Abbe number(v_(d)) of 39.5 and more.

[0090] The above-described optical glass preferably has the transitiontemperature (Tg) of 640° C. or less.

[0091] The above-described optical glass preferably has the transitiontemperature (Tg) of 630° C. or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0092]FIG. 1 is X-Y orthogonal coordinates in which X-axis is an Abbenumber (v_(d)) and Y-axis is a refractive index (n_(d)), and which showsthe range of optical constants (Abbe number (v_(d)) and refractive index(n_(d))) of the optical glass of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0093] Hereinafter, the reasons why the composition range in mass % ofeach components are limited as described above in an optical glass ofthe present invention will be explained below.

[0094] The SiO₂ component is an effective component in increasing theviscosity and improving the resistance to devitrification property ofthe optical glass of the present invention. However, when the contentthereof is less than 0.1%, it becomes difficult to obtain theabove-described effect efficiently. Further, when the glass contains theSiO₂ component excessively, it is not preferable that transitiontemperature (Tg) becomes high. In order to facilitate obtaining anoptical glass having sufficient resistance to devitrification propertyand desired low transition temperature (Tg), the content of SiO₂component is preferably 0.1% or more, more preferably 0.5% or more andthe most preferably 1% or more, and is preferably 8% or less, morepreferably 6% or less and the most preferably less than 5.5%.

[0095] The B₂O₃ component is an essential component as a glassconsisting oxide component according to the Lanthanum system opticalglass of the present invention. However, when its content is less than5%, the resistance to devitrification property becomes insufficient, andwhen it is not less than 20%, its chemical durability degrades.Therefore, the optical glass of the present invention containspreferably 5% or more, more preferably 6% or more and the mostpreferably 8% or more, and contains preferably less than 20%, morepreferably 19.5% or less and the most preferably 19% or less of B₂O₃component.

[0096] The La₂O₃ component is effective in increasing refractive indexand decreasing dispersion and is essential for the glass of the presentinvention having high refractive index and low dispersion. However, whenits content is 15% or less, it is difficult to keep the opticalconstants within the above-described specific area. On the other hand,when the glass contains the La₂O₃ component excessively, the resistanceto devitrification degrades. Therefore, in order to obtain particularlyfine resistance to devitrification property while the optical constantsof the glass is kept within the above-described specific area, the glasscontains preferably 15% or more, more preferably 18% or more, and themost preferably 20% or more, and contains preferably not more than 50%,more preferably less than 47%, and the most preferably not more than 45%of La₂O₃ component.

[0097] Gd₂O₃ component is effective in increasing refractive index anddecreasing dispersion of a glass. However, when its content is less than0.1%, the above-described effect is not sufficient, and on the contrarywhen the glass contains the Gd₂O₃ component excessively, resistance todevitrification property degrades. Therefore, the optical glass of thepresent invention contains preferably 0.1% or more, more preferably 0.5%or more and the most preferably 1% or more, and contains preferably 30%or less, more preferably less than 28% and the most preferably 25% orless of Gd₂O₃ component.

[0098] The Nb₂O₅ component is effective in increasing refractive indexand improving chemical durability and resistance to devitrificationproperty. On the contrary the resistance to devitrification propertydegrades when the glass contains the Nb₂O₅ component excessively.Therefore, in order to facilitate obtaining the above-described effect,the content thereof is preferably 0.1% or more. Furthermore, the contentthereof is preferably 8% or less, more preferably 7% or less and themost preferably 6% or less in order to obtain the fine resistance todevitrification property.

[0099] The GeO₂ component is effective component in increasingrefractive index and improving resistance to devitrification property.However, it is extremely expensive. Therefore, the glass of the presentinvention contains preferably 10% or less, more preferably 8% or lessand the most preferably 6% or less of GeO₂ component.

[0100] When the total content of the Gd₂O₃, the Nb₂O₅ and the GeO₂components is less than 10%, the resistance to devitrification propertyis insufficient, and when the total content is more than 30%, theresistance to devitrification property is insufficient all the same.Furthermore, since the materials of these three components are allexpensive, it is not preferable to add excessively from the viewpoint ofproduction cost. Therefore, according to the optical glass of thepresent invention, the total content of the Gd₂O₃, the Nb₂O₅ and theGeO₂ components is controlled preferably more than 10%, more preferably10.5% or more and. the most preferably 11% or more, and also controlledpreferably 30% or less, more preferably 28% or less and the mostpreferably 25% or less.

[0101] The Yb₂O₃ component is effective in increasing the refractiveindex and decreasing the dispersion of the glass. However, when theglass contains the Yb₂O₃ component excessively, the resistance todevitrification property degrades. Therefore, the optical glass of thepresent invention contains preferably 5% or less, more preferably 4% orless and the most preferably 3.5% or less of Yb₂O₃.

[0102] The TiO₂ component has the effects of regulating opticalconstants and improving resistance to devitrification property. However,when the glass contains the TiO₂ component excessively, the resistanceto devitrification property degrades. Therefore, the glass of thepresent invention contains preferably 1% or less, more preferably 0.8%or less and the most preferably 0.5% or less of TiO₂.

[0103] The ZrO₂ component has effects of regulating optical constantsand improving resistance to devitrification property and chemicaldurability. However, when the glass contains the ZrO₂ componentexcessively, the resistance to devitrification property degrades to thecontrary, and it becomes difficult to obtain desired low transitiontemperature (Tg). Therefore, the glass of the present invention containspreferably 8% or less, more preferably less than 8% and the mostpreferably 7.5% or less of ZrO₂. Particularly in order to facilitateobtaining the above-described effect, the content is preferably 0.1% ormore.

[0104] The Ta₂O₅ component has the effects of increasing refractiveindex and improving chemical durability and resistance todevitrification property. When the content thereof is 10% or less, noremarkable effect is seen, and when it is more than 25%, resistance todevitrification property degrades to the contrary.

[0105] Therefore, in order to obtain the optical glass of the presentinvention having enough chemical durability and resistance todevitrification property while the refractive index is kept in thedesired range, the content is preferably more than 10%, more preferably14% or more and the most preferably more than 19%. In addition, theupper limit is 25%.

[0106] The WO₃ component has the effects of regulating optical constantsand improving resistance to devitrification property. However, when theglass contains the WO₃ component excessively, the resistance todevitrification property and the light transmittance in shorterwavelength range of visible range decreases. Therefore, the glass of thepresent invention contains preferably 10% or less, more preferably 8% orless and the most preferably 6% or less of the WO₃. In particular, inorder to facilitate obtaining the glass having superior lighttransmittance in shorter wavelength range of visible range, it ispreferable that the content is less than 2% The ZnO component has alarge effect of lowering transition temperature (Tg). However, when theglass contains the ZnO component excessively, resistance todevitrification property degrades. Therefore, the glass of the presentinvention contains preferably 15% or less, more preferably 13% or lessand the most preferably 10% or less of ZnO. In addition, in order tofacilitate obtaining the above-described effect, it is preferable thatthe content is 0.1% or more.

[0107] The RO component, which is one or more kinds of the componentsselected from CaO, SrO and BaO, is effective in regulating opticalconstants. However, when the total content of CaO, SrO and BaOcomponents exceed 5%, the resistance to devitrification propertydegrades. Therefore, according to the glass of the present invention,the total content of CaO, SrO and BaO is preferably 5% or less, morepreferably 4% or less and the most preferably 3% or less.

[0108] The Li₂O component has the effects of lowering transitiontemperature (Tg) to a large degree and promoting the melting of themixed glass materials. However, when the content thereof is 0.5% orless, these effects are insufficient, and when the glass contains theLi₂O component excessively, resistance to devitrification propertydegrades to the contrary. Therefore, the glass of the present inventioncontains less than 3%, preferably 2.5% or less and the most preferably2% or less of the Li₂O. In order to facilitate obtaining the glasshaving even lower transition temperature (Tg), the content is more than0.5%, preferably 0.6% or more and the most preferably more than 1%.

[0109] The Sb₂O₃ component can be added to the molten glass fordefoaming. The content is preferably 1% and less, more preferably 0.8%and less and the most preferably 0.5% and less.

[0110] The Lu₂O₃ component has the effect of improving resistance todevitrification property when extremely small amount is added to theglass of the composition system according to the present invention.Since the material of the Lu₂O₃ component is extremely expensive, whenthe glass contains the Lu₂O₃ component excessively, the glass is notpractical because of its high production cost. Furthermore, theresistance to devitrification property degrades. Therefore, the glass ofthe present invention contains preferably less than 0.5%, morepreferably 0.45% or less and the most preferably 0.4% or less of Lu₂O₃.

[0111] The F component is effective in decreasing the dispersion of aglass, lowering transition temperature (Tg), improving resistance todevitrification property. In particular, as the inventors has found out,when the F component coexists with La₂O₃ component, the high refractiveindex and low dispersion optical glass is successful in having theoptical constants within the above-described specific area and the lowtransition temperature (Tg) at which precision mold pressing can beperformed. Therefore, F component is extremely important component inthe glass of the present invention.

[0112] A part or all of one kind or two or more kinds of metal oxide ofthe each above-described metal element is substituted with the metalfluoride thereof. When the total content of the fluoride calculated as Felement is less than 0.1%, the above-described effects of the Fcomponent are insufficient, and when it is more than 6%, it becomedifficult to obtain a uniform glass since the volatilization amount ofthe F component increases. In order to facilitate obtaining theabove-described effects, the glass of the present invention containspreferably 6% or less, more preferably 5.5% or less and the mostpreferably 5% or less, and contains preferably 0.1% or more, morepreferably 0.2% or more and the most preferably 0.5% or more of the Fcomponent.

[0113] Next, the components that should not be contained in the glass ofthe present invention will be explained. As described above, there areproblems in lead component that the lead contained glass easily fuseswith a metal molding die in precision mold pressing, and that some kindof environmental measures for the sake of unfavorable environmentaleffect of lead are required not only in manufacturing, but alsocoolworking such as polishing, disposal and the like of the glass.Therefore, the optical glass of the present invention should not containthe lead component, because the lead component has large environmentalburden.

[0114] Both of the cadmium and the thorium components have harmfuleffects to the environment and large environment burden. Therefore, theglass of the present invention should not contain them.

[0115] The P₂O₅ tends to have the effect of degrading resistance todevitrification property of the optical glass of the present invention.Therefore, it is not preferable that the glass contains P₂O₅ component.

[0116] With respect to the Y₂O₃, the inventors have found thatresistance to devitrification property of the optical glass of thepresent invention degrades unexpectedly when it contains the Y₂O₃component. Therefore, the glass should not contain it.

[0117] With respect to the TeO₂, when a glass material is melted in aplatinum crucible or a melting tank in which the part exposed to moltenglass is made of platinum, the platinum is alloyed the tellurium. Theheat resistance degrades in the alloyed part. The risk of alloyed partmelting to be a hole and molten glass flowing out is concerned.Therefore, the optical glass of the present invention should not containthe TeO₂.

EXAMPLES

[0118] The Examples of the present invention will be explained below.However, it is needless to say that the present invention is not limitedto such Examples.

[0119] Tables 1 to 6 show the composition of the examples according tothe glass of the present invention (No. 1 to No. 32) together with theirrefractive index (n_(d)), Abbe number (v_(d)) and transition temperature(Tg).

[0120] Tables 7 and 8 show the composition of the glass of thecomparative examples (No. A to No. H) together with their refractiveindex (n_(d)), Abbe number (v_(d)) and transition temperature (Tg).

[0121] Table 9 shows the results of a devitrification test of theexample of the present invention (No. 17, 25, 28 and 30) and thecomparative examples (No. I, J and K). TABLE 1 EXAMPLE COMPOSITION (mass%) No. 1 2 3 4 5 6 SiO₂ 5.49 5.39 5.39 1.77 1.56 5.44 B₂O₃ 12.51 10.7610.35 19.94 19.71 10.30 La₂O₃ 36.81 37.94 38.55 39.53 40.38 38.55 Gd₂O₃16.37 16.08 16.08 9.02 4.92 14.05 ZrO₂ 6.22 7.13 7.13 6.40 6.36 7.12Nb₂O₅ 1.04 1.02 2.04 1.54 5.09 2.04 Ta₂O₅ 19.38 19.54 18.02 15.86 15.7718.02 ZnO 1.04 1.02 1.02 5.12 5.09 1.02 Li₂O 1.04 1.02 1.32 0.72 1.021.32 Sb₂O₃ 0.10 0.10 0.10 0.10 0.10 0.10 GeO₂ 2.04 Gd₂O₃ + 17.41 17.1018.12 10.56 10.01 18.13 Nb₂O₅ + GeO₂ TOTAL 100 100 100 100 100 100AMOUNT F 3.63 1.78 1.78 4.02 2.96 1.78 nd 1.852 1.879 1.880 1.838 1.8511.878 νd 41.6 40.6 40.3 42.7 40.3 40.1 Tg (° C.) 610 630 613 592 583 621

[0122] TABLE 2 EXAMPLE COMPOSITION (mass %) No. 7 8 9 10 11 12 SiO₂ 5.441.56 5.44 5.44 5.44 5.44 B₂O₃ 10.30 16.68 10.30 10.30 10.30 10.30 La₂O₃38.05 38.15 38.05 38.05 38.05 38.05 Gd₂O₃ 14.05 10.18 13.03 13.74 10.7412.74 Yb₂O₃ 3.00 TiO₂ 1.00 ZrO₂ 6.10 6.35 7.12 6.10 6.10 6.10 Nb₂O₅ 2.555.09 2.55 2.55 2.55 2.55 Ta₂O₅ 20.05 15.78 20.05 20.36 20.36 20.36 ZnO5.09 Li₂O 1.32 1.02 1.32 1.32 1.32 1.32 Sb₂O₃ 0.10 0.10 0.10 0.10 0.100.10 GeO₂ 2.04 2.04 2.04 2.04 2.04 Gd₂O₃ + 18.64 15.27 17.62 18.33 15.3317.33 Nb₂O₅ + GeO₂ TOTAL 100 100 100 100 100 100 AMOUNT F 1.78 1.78 1.781.78 1.78 1.78 nd 1.881 1.874 1.885 1.882 1.880 1.887 νd 39.7 40.1 39.339.4 39.5 39.1 Tg (° C.) 630 595 628 627 625 629

[0123] TABLE 3 EXAMPLE COMPOSITION (mass %) No. 13 14 15 16 17 SiO₂ 5.445.44 5.44 5.44 4.09 B₂O₃ 10.30 10.30 10.30 10.30 13.23 La₂O₃ 38.05 38.0538.05 38.05 35.50 Gd₂O₃ 10.74 10.74 10.74 10.74 7.13 ZrO₂ 6.10 6.10 6.106.10 6.35 Nb₂O₅ 2.55 2.55 2.55 2.55 5.09 Ta₂O₅ 18.36 20.36 20.36 20.3620.36 WO₃ 5.00 ZnO 5.09 CaO 3.00 SrO 3.00 BaO 3.00 Li₂O 1.32 1.32 1.321.32 1.02 Sb₂O₃ 0.10 0.10 0.10 0.10 0.10 GeO₂ 2.04 2.04 2.04 2.04 2.04Gd₂O₃ + Nb₂O₅ + GeO₂ 15.33 15.33 15.33 15.33 14.26 TOTAL AMOUNT 100 100100 100 100 F 1.78 1.78 1.78 1.78 1.78 nd 1.881 1.874 1.874 1.874 1.882νd 39.0 39.5 39.5 39.5 37.9 Tg (° C.) 622 619 620 618 604

[0124] TABLE 4 EXAMPLE COMPOSITION (mass %) No. 18 19 20 21 22 SiO₂ 5.445.44 5.45 1.56 2.67 B₂O₃ 12.34 12.34 9.28 14.14 14.65 La₂O₃ 37.03 32.0339.06 39.67 37.54 Gd₂O₃ 13.03 13.03 11.71 7.13 5.09 ZrO₂ 6.10 6.10 6.106.35 6.35 Nb₂O₅ 2.55 2.55 2.55 5.09 5.09 Ta₂O₅ 19.34 19.34 20.36 15.7820.36 ZnO 3.05 8.05 5.09 5.09 Li₂O 1.02 1.02 1.32 1.02 1.02 Sb₂O₃ 0.100.10 0.10 0.10 0.10 GeO₂ 4.07 4.07 2.04 Gd₂O₃ + Nb₂O₅ + GeO₂ 15.58 15.5818.33 16.29 12.22 TOTAL AMOUNT 100 100 100 100 100 F 1.78 1.78 1.78 1.781.78 nd 1.873 1.861 1.884 1.882 1.881 νd 40.0 39.8 39.2 38.6 38.0 Tg (°C.) 612 580 630 591 601

[0125] TABLE 5 EXAMPLE COMPOSITION (mass %) No. 23 24 25 26 27 SiO₂ 5.395.39 5.39 5.45 5.45 B₂O₃ 12.28 12.28 12.28 10.71 12.34 La₂O₃ 37.94 37.9437.94 37.33 37.03 Gd₂O₃ 8.45 8.24 8.65 13.03 13.03 ZrO₂ 6.11 6.11 6.116.11 6.11 Nb₂O₅ 1.02 1.02 1.02 2.04 2.04 Ta₂O₅ 19.85 19.85 19.85 19.5419.85 ZnO 5.60 5.60 5.60 5.09 3.05 Li₂O 1.02 1.02 1.02 0.61 1.02 Sb₂O₃0.10 0.10 0.10 0.10 0.10 GeO₂ 2.04 2.04 2.04 Lu₂O₃ 0.20 0.41 Gd₂O₃ +Nb₂O₅ + GeO₂ 11.50 11.30 11.70 15.06 15.06 TOTAL AMOUNT 100 100 100 100100 F 1.78 1.78 1.78 1.78 1.78 nd 1.864 1.864 1.864 1.884 1.867 νd 40.640.6 40.6 39.6 40.4 Tg (° C.) 607 607 608 614 610

[0126] TABLE 6 EXAMPLE COMPOSITION (mass %) No. 28 29 30 31 32 SiO₂ 5.455.45 5.45 3.41 5.45 B₂O₃ 12.34 12.01 13.54 15.57 13.23 La₂O₃ 37.23 32.5732.06 33.59 32.88 Gd₂O₃ 10.99 10.18 10.18 10.18 10.18 ZrO₂ 6.11 6.116.11 6.11 6.11 Nb₂O₅ 1.53 2.04 3.05 1.53 2.54 Ta₂O₅ 19.85 19.85 19.8519.85 19.85 WO₃ 0.00 2.44 2.44 2.44 2.44 ZnO 5.60 5.60 5.60 5.60 5.60Li₂O 0.81 1.63 1.63 1.63 1.63 Sb₂O₃ 0.10 0.10 0.10 0.10 0.10 GeO₂ 2.04Lu₂O₃ Gd₂O₃ + Nb₂O₅ + GeO₂ 12.52 14.25 13.23 11.70 12.72 TOTAL AMOUNT100 100 100 100 100 F 1.78 1.78 1.78 1.78 1.78 nd 1.867 1.859 1.8571.853 1.855 νd 40.4 39.3 39.0 40.0 39.4 Tg (° C.) 608 577 576 570 579

[0127] TABLE 7 COMPARATIVE EXAMPLE COMPOSITION (mass %) No. A B C D E FG SiO₂ 12.00 5.00 6.00 4.50 4.49 3.00 3.00 B₂O₃ 11.90 16.30 11.00 15.5014.97 16.30 17.00 Al₂O₃ 1.00 Y₂O₃ 8.60 5.00 2.00 2.00 La₂O₃ 32.00 38.4033.00 35.00 35.43 45.00 46.40 Gd₂O₃ 33.00 9.60 25.00 10.00 7.98 ZrO₂2.00 5.00 4.00 5.00 4.99 5.00 3.00 Nb₂O₅ 3.00 7.00 Ta₂O₅ 7.10 15.70 9.0025.00 21.16 11.00 9.30 WO₃ 1.40 7.00 5.00 4.99 11.70 12.30 ZnO 1.80 1.00Li₂O 1.00 0.20 PbO 3.99 Yb₂O₃ 2.00 TOTAL AMOUNT 100 100 100 100 100 100100 F₂ 3.63 nd 1.804 1.851 1.882 1.872 1.875 1.881 1.884 νd 47.2 42.940.6 38.6 37.8 37.7 36.4 Tg (° C.) 673 709 739 701 655 675 674

[0128] TABLE 8 COMPARATIVE EXAMPLE COMPOSITION (mass %) No. H SiO₂ 2.00B₂O₃ 10.00 La₂O₃ 40.00 Gd₂O₃ 10.00 Lu₂O₃ 5.00 ZrO₂ 2.50 Nb₂O₅ 1.00 Ta₂O₅18.50 ZnO 1.00 GeO₂ 10.00 TOTAL AMOUNT 100 nd 1.894 νd 40.9 Tg (° C.)707

[0129] The optical glasses of the examples of the present invention (No.1 to No. 32) shown in Tables 1 to 6 were obtained in the followingprocesses. General optical glasses materials such as oxides, hydroxides,carbonates, nitrates, fluorides and the like were weighed by their massproportions according to be the composition of the examples shown inTables 1 to 6. The mixed glass materials were installed to a platinumcrucible of 300 cc volume and were melted, clarified and stirred at1200° C. to 1400° C. for 3 to 6 hours depending on their meltabilitybased on the compositions. The homogenized molten glasses were cast intoa metal molding die or the like and annealed.

[0130] As shown in Tables 1 to 6, all the optical glasses of theexamples of the present invention (No. 1 to No. 32) have the opticalconstants (refractive index (n_(d)) and Abbe number (v_(d))) within theabove-described specific area and the transition point (Tg) in a rangeof 550 to 650° C. Thus, their optical glasses are suitable for the glasspreform material for precision mold pressing and precision moldpressing.

[0131] On the contrary, as shown in Tables 7 and 8, the glass of thecomparative example No. A (the example 6 of the above-describedPublication 1) contains fluorine, but has transition temperature (Tg) ofmore than 650° C. Therefore, it is difficult to be applied to precisionmold pressing. Further, the optical constants are not within theabove-described specific area. With respect to the glasses of thecomparative example No. B (the example 1 of the above-describedPublication 2), the comparative example No. C (the example 2 of theabove-described Publication 2), the comparative example No. D (theexample 2 of the above-described Publication 3), the comparative exampleNo. E (the example 5 of the above-described Publication 3), thecomparative example No. F (the example 2 of the above-describedPublication 4), the comparative example No. G (the example 3 of theabove-described Publication 4) and the comparative example No. H (theexample 4 of the above-described Publication 5), all of them have theoptical constants within the above-described specific area. However,since their transition temperature (Tg) is more than 650° C., it isdifficult to be applied to precision mold pressing.

[0132] Next, the devitrification test will be explained. 50cc ofexamples No. 17, 25, 28 and 30 and No. I, J, K and L having thecompositions of described below were respectively installed into aplatinum crucible and melted in a electric furnace at 1300° C. for 1hour in order to be the complete glass molten. Successively thetemperature was lowered and kept at 1150° C., 1180° C. or 1200° C. for2hours, and the crucibles were taken out from the furnace. The conditionof the glass moltens in a crucible are visually observed.

[0133] The composition of the comparative example 17 is based on that ofthe example 17, except 4.58 mass % of La₂O₃ component was substitutedwith Y₂O₃. The comparative examples J, K, and L had same compositions ofthe example 23 and 28 of the above-described Publication 7 (theircompositions shown in Publication 7 are converted from mol% mass%) andthe example 4 of the above-described Publication 6 respectively.

[0134] A circle mark (◯) indicates that devitrification was generated,and a cross mark (x) indicates that devitrification was not generated.As shown in Table 9, the molten glasses of the example of the presentinvention are superior in resistance to devitrification property to thecomparative example I to L. Particularly, from the example 17 and thecomparative example I, it is found that the resistance todevitrification property degrades when the glass contains Y₂O₃. TABLE 9COMPARATIVE EXAMPLE EXAMPLE 17 25 28 30 I J K L SiO₂ 4.09 5.39 5.39 5.454.09 5.99 5.95 7.30 B₂O₃ 13.23 12.28 12.34 13.54 13.23 10.75 10.33 9.40Y₂O₃ 4.58 La₂O₃ 35.50 37.94 37.23 32.06 30.92 25.97 25.79 45.80 Gd₂O₃7.13 8.65 10.99 10.18 7.13 28.90 28.69 10.00 ZrO₂ 6.35 6.11 6.11 6.116.35 3.07 3.66 7.20 Nb₂O₅ 5.09 1.02 1.53 3.05 5.09 1.20 Ta₂O₅ 20.3619.85 19.85 19.85 20.36 9.90 10.93 13.90 WO₃ 2.44 6.93 6.89 ZnO 5.09 5.65.6 5.6 5.09 7.3 6.44 5.00 Li₂O 1.02 1.02 0.81 1.63 1.02 1.19 1.33 Sb₂O₃0.10 0.10 0.10 0.10 0.10 0.20 GeO₂ 2.04 2.04 2.04 TOTAL 100 100 100 100100 100 100 100 F 1.78 1.78 1.78 1.78 1.78 DEVITRIFICATION TEST 1200° C.no ∘ ∘ ∘ no x x x data data 1180° C. no ∘ ∘ ∘ no x x x data data 1150°C. ∘ x ∘ ∘ x x x x

[0135] The entire disclosure of Japanese Patent Application No.2002-365851 filed on Dec. 17, 2002 including specification, claims,drawings and summary is incorporated herein by reference in itsentirety.

What is claimed is:
 1. An optical glass having; a refractive index(n_(d)) and an Abbe number (v_(d)) which are within an area surroundedby straight lines that are drawn by connecting point A (n_(d)=1.835,v_(d)=46.5), point B (n_(d)=1.90, v_(d)=40.0), point C, (n_(d)=1.90,v_(d)=35.0) and point D (n_(d)=1.835, v_(d)=38.0) in a sequence of A, B,C, D and A as border lines in x-y orthogonal coordinates shown in FIG.1, in which X-axis is the Abbe number (v_(d)) and Y-axis is therefractive index (n_(d)), the area including the border lines: and theoptical glass comprising: 0.1 to 8 mass % of SiO₂; 5 to less than 20mass % of B₂O₃; 15 to 50 mass % of La₂O₃; 0.1 to 30 mass % Gd₂O₃, 0 to10 mass % of GeO₂ and 0 to 8 mass % of Nb₂O₅, where a total content ofGd₂O₃, GeO₂ and Nb₂O₅ is more than 10 mass % to 30 mass %; 0 to 5 mass %of Yb₂O₃; 0 to 1 mass % of TiO₂; 0 to 8 mass % of ZrO₂; more than 10 to25 mass % of Ta₂O₅; 0 to 10 mass % of WO₃; 0 to 15 mass % of ZnO; 0 to 5mass % of RO, where RO is one or more kinds of oxides selected from CaO,SrO and BaO; more than 0.5 to less than 3 mass % of Li₂O; 0 to 1 mass %of Sb₂O₃; and 0.1 to 6 mass % in a the total content of fluorides ofabove-described metal elements as F element with which a part or all ofone or more kinds of oxides of above-described metal elements aresubstituted; wherein the optical glass is free from cadmium, thorium,Y₂O₃, P₂O₅, and TeO₂, and the optical glass has a transition temperature(Tg) of 550 to 650° C.
 2. The optical glass as claimed in claim 1,comprising 0.1 to less than 5.5 mass % of SiO₂.
 3. The optical glass asclaimed in claim 1, comprising more than 1 to less than 3 mass % ofLi₂O.
 4. The optical glass as claimed in claim 1, having the refractiveindex (n_(d)) of less than 1.875.
 5. The optical glass as claimed inclaim 1, having the refractive index (n_(d)) of 1.875 or more.
 6. Theoptical glass as claimed in claim 1, having the refractive index (n_(d))of more than 1.85.
 7. The optical glass as claimed in claim 1, havingthe Abbe number (v_(d)) of less than 39.5.
 8. The optical glass asclaimed in claim 1, having the Abbe number (v_(d)) of 39.5 or more. 9.The optical glass as claimed in claim 1, having the transitiontemperature (Tg) of 640° C. or less.
 10. The optical glass as claimed inclaim 1, having the transition temperature (Tg) of 630° C. or less. 11.The optical glass as claimed in claim 2, comprising more than 1 to lessthan 3 mass % of Li₂O.
 12. The optical glass as claimed in claim 4,having the Abbe number (v_(d)) of 39.5 or more.
 13. The optical glass asclaimed in claim 12, having the refractive index (n_(d)) of more than1.85.
 14. The optical glass as claimed in claim 5, having the Abbenumber (v_(d)) of less than 39.5.
 15. An optical glass having; arefractive index (n_(d)) and an Abbe number (v_(d)) which are within anarea surrounded by straight lines that are drawn by connecting point A(n_(d)=1.835, v_(d)=46.5), point B (n_(d)=1.90, v_(d)=40.0), point C,(n_(d)=1.90, v_(d)=35.0) and point D (n_(d)=1.835, v_(d)=38.0) in asequence of A, B, C, D and A as border lines in x-y orthogonalcoordinates shown in FIG. 1, in which X-axis the Abbe number (v_(d)) andY-axis is the refractive index (n_(d)), the area including the borderlines: and the optical glass comprising: 0.1 to 8 mass % of SiO₂; 5 toless than 20 mass % of B₂O₃; 15 to 50 mass % of La₂O₃; 0.1 to 30 mass %Gd₂O₃, more than 10 to 25 mass % of Ta₂O₅; and more than 0.5 to lessthan 3 mass % of Li₂O; and 0 to 10 mass % of GeO₂ and/or 0 to 8 mass %of Nb₂O₅, where a total content of Gd₂O₃, GeO₂ and Nb₂O₅ is more mass %to 30 mass %; and/or 0 to 5 mass % of Yb₂O₃; and/or 0 to 1 mass % ofTiO₂; and/or 0 to 8 mass % of ZrO₂; and/or 0 to 10 mass % of WO₃; and/or0 to 15 mass % of ZnO; and/or 0 to 5 mass % of RO, where RO is one ormore kinds of oxides selected from CaO, SrO and BaO; 0 to 1 mass % ofSb₂O₃; and/or 0 to less than 0.5 mass % of Lu₂O₃; and 0.1 to 6 mass % inthe total content of fluorides of above-described metal elements as Felement with which a part or all of one or more kinds of oxides ofabove-described metal elements are substituted; wherein the opticalglass is free from cadmium, thorium Y₂O₃, P₂O₅ and TeO₂, and the opticalglass has a transition temperature (Tg) of 550 to 650° C.
 16. Theoptical glass as claimed in claim 15, comprising 0.1 to less than 5.5mass % of SiO₂.
 17. The optical glass as claimed in claim 15, comprisingmore than 1 to less than 3 mass % of Li₂O.
 18. The optical glass asclaimed in claim 15, having the refractive index (n_(d)) of less than1.875.
 19. The optical glass as claimed in claim 15, having therefractive index (n_(d)) of 1.875 or more.
 20. The optical glass asclaimed in claim 15, having the refractive index (n_(d)) of more than1.85.
 21. The optical glass as claimed in claim 15, having the Abbenumber (v_(d)) of less than 39.5.
 22. The optical glass as claimed inclaim 15, having the Abbe number (v_(d)) of 39.5 or more.
 23. Theoptical glass as claimed in claim 15, having the transition temperature(Tg) of 640° C. or less.
 24. The optical glass as claimed in claim 15,having the transition temperature (Tg) of 630° C. or less.
 25. Theoptical glass as claimed in claim 16, comprising more than 1 to lessthan 3 mass % of Li₂O.
 26. The optical glass as claimed in claim 18,having the Abbe number (v_(d)) of 39.5 or more.
 27. The optical glass asclaimed in claim 26, having the refractive index (n_(d)) of more than1.85.
 28. The optical glass as claimed in claim 19, having the Abbenumber (v_(d)) of less than 39.5.